Yeast: is an unicellular saprophyc fungus present in the air and on the surface of fruit. Yeat causes the spoilage of fruit, jam, honey... is used in the production of bread, beer, wine... it feeds on carbohydrate foods, flourishes at 25C-30C. Is killed above 60C, requires moisture for growth, it can live with or without oxygen, requires time to grow. reproduce asexually (budding): cell devolps a bud, nucleous moves towards the bud, nucleous divides in two, a wall develops and divided the bud from the parent cell, bud separate from cell. Granular cytoplasm, food vacuoles, vacuole, cell wall. 

Food preservation: long term storage, prevents enzyme activity in food, inhibits the growh of microbes, prevent microbes from re-entering by sealing the food, maintains as much nutritive value, colour, flavour and texture. Methods: freezing, heat treatments, chemical preservation, dehydration.

Freezing: use of low temperatures, unsuitable for enzyme activity and microbial growth. Micro-organisims are inactivated not killed, ice prevents microbial multiplication, wrapping food prevents re-entry of microorganisms. Quick: -25C, small ice crystalls causing less damage to cell walls, nutritive value, texture, colour, flavour better retained. Slow: -6C /-18C, large ice crystals... greater loss nutritive... packaging: freezer bags, polythene boxes, aluminium containers. Blanching: scalding food in boil water to inactive enzymes, plunging into water stop cooking process. 

Heat treatments: fruit vegetables very high temperatures: jam making: fruit is boiled with high proportion of sugar. Heat kill microbes and enzymes, 65% sugar is used, acts as a preservative, Pectin and acid help jam to set. Problem: crystallisation, jam not set, fermentation. Chutney-making: boiling fruit and vegetable with vinegar and spices.

Chemical preservation: Jam making-sugar form a concentrated solution which dehydrates microbial cells, chutney making-sugar and vinegar lowers the ph to an unacceptable level for microbial growh. Pickling- vinegar and salt forms a concentrated solution which dehydrates microbial cells

Dehydration: herbs can be dried by tying them and dry in a hot press. Microorganisms and enzymes require moisture are inactive without it.